An investigation by Guardian Films [click for video], with funding from CI, has found a catalogue of environmental and social damage caused by intensive tropical fruit production in Costa Rica, from where three-quarters of pineapples sold in the UK come.

Our findings include evidence that:

• The constant use of agrochemicals has led to contamination of drinking-water supplies to communities around the plantations. One group of villages, bordering plantations that either supply or are owned by Del Monte, has been forced to collect water from tankers for more than three years.

• Repeated chemical accidents have inflicted serious damage on the local environment. In June this year, a fire at a chemical dump used by the multinational trader Dole caused a toxic cocktail to spill into the nearby river. Fish and other wildlife including crocodiles were killed along miles of waterways. The pesticide involved is sprayed by Dole on its bananas. Dole told us that it was co-operating with the authorities investigating the accident.

• Price cuts in European supermarkets have led to wages being drastically cut by Grupo Acon, workers said.

• Efforts to join independent trade unions to improve conditions are said to have been met with repression and mass sackings on Del Monte and Grupo Acon plantations.

Consumers International, which acts as an umbrella group for independent consumer organisations across Europe, says its own research has shown that conditions in Costa Rica's tropical fruit industry are unacceptable. It accuses supermarkets such as Tesco and Asda of complicity despite their public commitments to source food responsibly.

"Their positive intentions are being undermined by retail price cuts and aggressive procurement practices that lead to cuts in wages and insufficient resources to improve working practices," said Catherine Nicholson, CI's programme co-ordinator. "Consumers want low prices but not at the cost of fair living conditions for producers."

Del Monte, which exports 50% of Costa Rica's pineapples, told us it had strict controls on chemicals to minimise accidents. It said that the government was monitoring the drinking water situation. It also said that all its workers were free to join unions. Grupo Acon did not respond to the allegations in our film but later said that it had strict protocols to deal with accidents. Dole declined to comment on conditions at its supplier. Fyffes, supplier to Asda of some Grupo Acon fruit, told us that it had checked in audits and was satisfied that all its suppliers had an open attitude to unions and did not discriminate against union members. Asda told us that it had reiterated to suppliers the high ethical standards expected of them.

Tesco told us that it was already actively engaged in improving standards: "After two years working with suppliers in Costa Rica, we have brokered agreement to invite independent labour experts to review labour practices and identify ways to strengthen them. That project is underway and is examining issues including pay, labour relations, seasonal labour and health and safety."